Due to the exhaustion of traditional energy sources together with an increase in the global energy demand, impetus is currently being given to the development of alternative energy sources. Among them, biomass is renewable quantitative biological resource that attracts a great deal of attention.
Among biomass-based industrial raw materials, isosorbide (C6H10O4) that is prepared by dehydration of sorbitol (C6H14O6) attracts attention as an environmentally friendly raw material for preparing polycarbonate (PC) as a substitute for bisphenol A (BPA), an epoxy monomer or an environmentally friendly plasticizer. Namely, isosorbide, a material that can be obtained by simple dehydration of sorbitol, is attracting attention as a monomer required for synthesis of next-generation, high-performance, environmentally friendly materials that can replace conventional polymer products, and many studies thereon have been conducted.
Environmentally friendly materials generally show poor properties compared to petrochemical-based materials, whereas isosorbide advantages in that it is environmentally friendly and, at the same time, shows excellent properties compared to conventional petrochemical-based materials. In addition, isosorbide may be used as an additive that can make plastic materials stronger and tougher, and that is also used as an agent for treating cardiac diseases by being boded to nitrate.
When D-glucose obtained from biomass by pretreatment is hydrogenated in the presence of a catalyst, sorbitol is produced. Isosorbide is produced by double dehydration of sorbitol. This cyclization reaction is influenced by various reaction conditions, including temperature, pressure, solvent, catalyst, etc.
Currently, as a method of preparing isosorbide from sorbitol, a process is widely used in which sulfuric acid is used as a catalyst and a reaction is carried out under a reduced pressure of about 10 mmHg. However, when a liquid strong acid catalyst such as sulfuric acid is used, a reactor is easily corroded, and for this reason, an expensive reactor should be used. In addition, a large amount of energy is continuously consumed to maintain a high vacuum level of about 10 mmHg, and thus the operating cost for the reaction is high, and high-reliability continuous vacuum reactor is also not easy to manufacture.
In recent years, in an attempt to solve the problems of such vacuum reactions, methods of carrying out reactions under high-temperature and high-pressure conditions have been reported.
U.S. Pat. No. 7,420,067 discloses a process for producing anhydrosugar alcohol, comprising: heating sugar alcohol or monoanhydrosugar alcohol to a temperature of 150° C. to 350° C. in the presence of an acidic catalyst; and pressurizing the sugar alcohol or monoanhydrosugar alcohol to a pressure of 130 psi to 2000 psi. U.S. Pat. No. 6,013,812 discloses a process for producing anhydrosugar alcohol, comprising reacting a polyol in the presence of an acidic catalyst and a hydrogenating catalyst at a temperature of at least 100° C. and a hydrogen pressure of 1 MPa to 20 MPa. These high-pressure reactions aim to increase isosorbide purity rather than the yield of isosorbide, and have a disadvantage in that the yield of isosorbide is lower than that in the vacuum reactions. U.S. Pat. No. 7,420,067 shows a yield of 41.4 mol % to 59.8 mol %, and U.S. Pat. No. 6,013,812 shows a yield of up to 46 mol %.
Meanwhile, the cost of the raw material sorbitol accounts for about 50% or more of the total production cost of isosorbide. For this reason, in order for the high-pressure reaction to be used commercially, the high-pressure reaction is required to have a yield similar to that of the vacuum reaction.
Accordingly, the present inventors have found that, when an effective catalyst, which is less acidic than sulfuric acid and can suppress side reactions at high temperature, is selected and used in a high-pressure reaction that converts sorbitol to isosorbide, the yield of isosorbide in the high-pressure reaction can be increased to a level similar to that in a vacuum reaction, thereby completing the present invention.